1. Field of the Invention
The invention relates to a ball screw device, and a power transfer mechanism and an electric power steering apparatus including the ball screw device, and a method for manufacturing the ball screw device.
2. Description of Related Art
As a steering apparatus for vehicles, a rack parallel electric power steering apparatus is known. The electric power steering apparatus includes a ball screw device attached to a rack shaft, a motor, and a speed reducer coupling an output shaft of the motor and the ball screw device together. The motor output shaft is disposed parallel to the rack shaft.
The ball screw device has a nut having a screw-thread, via a plurality of balls, that mates with a spiral groove formed on an outer peripheral surface of the rack shaft. The screw-thread is a spiral groove facing the groove on the rack shaft is formed on an inner peripheral surface of the nut. A space enclosed by the groove in the nut and the groove on the rack shaft forms a rolling path through which balls roll. Furthermore, the nut has a pair of mounting holes penetrating the nut from the groove on the inner peripheral surface to the outer peripheral surface. The mounting holes are arranged so as to be separated from each other in an axial direction of the nut. The mounting holes are in communication with each other via a recessed communication groove formed on the outer peripheral surface of the nut. Furthermore, a deflector is installed in each of the mounting holes. The deflectors installed in the mounting holes and the communication groove provide a circulating path that connects two positions in the rolling path. The circulating path and the rolling path enable the balls to circulate endlessly through the rolling path.
The above-described speed reducer has a driving pulley integrally assembled to the output shaft of the motor, a driven pulley assembled to the nut, and an endless belt wound around both pulleys. In the rack parallel electric power steering apparatus, when the output shaft of the motor rotates, the rotating motion of the output shaft is transmitted to the nut via the driving pulley, the belt, and the driven pulley. Thus, the nut rotates relative to the rack shaft, and the balls circulate endlessly through the rolling path. Through the endless circulation of the balls, an axial assist force is applied to the rack shaft, thereby assisting steering operations of a driver.
In an electric power steering apparatus described in Japanese Patent No. 4807655, the driven pulley is assembled to the outer peripheral surface of the nut to close an opening portion of the communication groove on the outer peripheral surface of the nut. In such a structure, given that the driven pulley is not assembled to the outer peripheral surface of the nut, the ball may fall through the communication groove. Thus, for example, when the ball screw device is evaluated for quality check during the process of manufacturing the electric power steering apparatus, an operation of installing and fixing the driven pulley to the outer peripheral surface of the nut is needed. The need for such an operation correspondingly increases the number of manufacturing steps, reducing operational efficiency. Such a problem occurs not only in the ball screw device used for the electric power steering apparatus but also possibly in ball screw devices used for other apparatuses.